Those skilled in the art know silicon mechanical or micromechanical parts. These are, for example, timepiece parts such as wheels, hands, balance springs, levers or any other essentially flat part. These parts have interesting mechanical properties, such as good resistance to wear or a low friction coefficient. They are generally manufactured from silicon on insulator type substrates, formed of a silicon wafer, an embedded insulating layer such as an oxide or sapphire layer, and a superficial silicon layer with a thickness of several tens to several hundreds of microns. Manufacture includes several steps implementing processes developed for the integrated circuit industry. These processes are for making parts with complex shapes in the plane, with greater precision than the precision obtained by stamping.
The main manufacturing steps for a mechanical silicon part from a substrate including a superficial silicon layer, of the “silicon-on-insulator” type, are schematically illustrated in FIG. 1. In order to facilitate the diagram and the reading thereof, scale has deliberately not been respected.
FIG. 1a: A substrate 10 formed of a silicon wafers 12, of an oxide embedded layer 14 and a superficial silicon layer 16, is oxidised on its entire face A superficial oxide layer 18, extending over the entirety of the superficial silicon layer 16 and typically having a substantially uniform thickness of 1 micron, is thus obtained. The oxidisation method is generally a wet or dry thermal method. In a variant, this could be an oxidisation by deposition method, of the physico-chemical type, such as a plasma enhanced chemical vapour deposition.
FIG. 1b: The superficial oxide layer 18 is structured by photolithography so as to form an oxide mask 20. This structuring step includes a first sub-step of layering a photosensitive resin, a second sub-step of local exposure of the resin to ultraviolet rays, a third sub-step of hardening the resin, a fourth sub-step of developing the resin, a fifth sub-step of etching the oxide through the resin mask, by a wet or dry process, then a sixth sub-step of cleaning the resin. All of these sub-steps are conventional, and will not be described in detail, since they are well known to those skilled in the art.
It will be noted that the resin layering sub-step ends in an operation of removing the resin on the periphery of the superficial oxide layer 18. This operation is achieved in particular by using a jet of solvent directed at the edge of substrate 10, over a width of 2 to 5 millimeters. It has the effect of exposing the periphery of the superficial oxide layer 18 for the etch, and thus of stripping off the periphery of superficial silicon layer 16, at the end of etching. Substrate 10 thus has, after mask 20 has been formed, a central zone 21 essentially covered with oxide, and a peripheral zone 22, from which the silicon is stripped away.
The operation of removing the resin from the periphery of the substrate is conventional within the field of integrated circuit manufacture. The purpose thereof is to remove from this zone, all of the deposited layers, in order to prevent the formation of an edge bead formed of the stack of various layers. This type of bead is the cause of problems of pealing, short-circuits, gripping and maintaining substrates.
The operation of removing resin from the periphery of the substrate is not necessary in mechanical part manufacture. Indeed, the number of manufacturing steps is, in such case, reduced and does not give rise to the formation of a bead. However, the resin removal operation cannot easily be omitted for the following reasons. Mechanical silicon parts are generally manufactured in integrated circuit manufacturing lines. The methods used are suited for manufacturing integrated circuits, and it is time-consuming and expensive to alter such methods when dealing with batches of mechanical parts. Moreover, the presence of resin on the periphery of the substrates raises serious contamination problems. Indeed, contact with the substrate gripping devices occurs via the periphery of the substrate. If the latter is covered with resin, the gripping device becomes loaded with resin and the latter gradually accumulates until it becomes unstuck and is deposited indiscriminately on the substrates. The parts contaminated by the resin are defective and have to be rejected. For these reasons, the resin removal operation is obligatory at the end of the resin layering, for batches of mechanical parts, just as for batches of integrated circuits.
FIG. 1c: The superficial silicon layer 16 is etched through the oxide mask 20, as far as the embedded oxide layer 14, so as to form mechanical silicon parts 24, including a top face 26, a flank 28 and a bottom face 30. At this stage of manufacture, top and bottom faces 26 and 30 are still secured respectively to superficial oxide layer 18 and embedded oxide layer 14.
The etch method used for etching the superficial silicon layer is highly selective, i.e., it etches the silicon strongly and the oxide weakly, so that definition of the silicon parts is optimum. It is also highly anisotropic, i.e., able to etch superficial silicon layer 16 perpendicular to the plane of substrate 10 and not parallel thereto. An anisotropic etch is indispensable for flanks 28 of mechanical parts 24 to be perpendicular to their top and bottom faces 26 and 30.
This selective and anisotropic etch method is generally of the physico-chemical type. Reactive ion etching is the most frequently used. This is a plasma assisted gas phase etch. The gas is generally a fluorinated gas, like SF6, CF4 or CHF3. The effect of the plasma is to create reactive species and to accelerate them perpendicularly to substrate 10. In FIG. 1c, the plasma is represented by arrows directed perpendicular to substrate 10, referenced 30.
In the standard mechanical part manufacturing method, the stripped off peripheral silicon zone 22 is exposed to plasma 30 for the whole of the etch. This zone 22 forms, opposite plasma 30, a chemical discontinuity relative to central zone 21, covered in oxide. An inhomogeneity in plasma 30 is then created in proximity to the transition between central zone 21 and peripheral zone 22. The effect of this local inhomogeneity is to interfere with the etch in proximity to peripheral zone 22, and in particular the anisotropic nature thereof. It will be observed that the mechanical parts 24 located in proximity to peripheral zone 22, have flanks 28 that are not perpendicular to the top and bottom faces 26 and 30, but oblique, either recessed in or opening out these parts have to be rejected because they are liable to wear prematurely and in a non uniform manner. Eventually, this wear could cause a violent rupture of the part.
FIG. 1d: The top 18 and embedded 14 oxide layers are removed by wet means, generally by etching in a concentrated hydrofluoric acid bath. Silicon parts 24 are thus released from substrate 10.
The method of manufacturing silicon mechanical parts thus described, is the state of the art method. It leads to the rejection of peripheral parts that constitute a percentage of the order of 10 to 30 percent of the total number of parts, depending upon the size of the parts.